Asian Journal of Plant Sciences

Year: 2021 | Volume: 20 | Issue: 2 | Page No.: 183-195
DOI: 10.3923/ajps.2021.183.195

Edaphic and Climatic Factors Affecting Phenology of Naturally Growing Calotropis procera in Semi-arid Regions of Kenya

Brexidis Mandila , Kenneth Odhiambo, Alice Muchugi, Daniel Nyamai and Agnes Gachuiri

Abstract: Background and Objective: Cultivating Calotropis procera for fiber supply to the textile industry can improve the livelihoods of communities in arid and semi-arid regions. This study determined edaphic and climatic factors affecting phenological traits of C. procera in the semi-arid regions of Kenya. Materials and Methods: Repeated measure research design was used with multistage sampling technique to monitor activity indices, number of flowers and fruits and phenophase intensities. Climatic and edaphic factors of study sites were also monitored. Data was analyzed using linear, Poisson log linear regression based on Generalized Estimation Equation (GEE) and Mixed Analysis of Variance (ANOVA). Results: High Soil Organic Carbon (OC) content (3%) and exchangeable Na (112.5 ppm) at (0-20) cm soil depth were recorded in Tharaka. High mean monthly rainfall (160.37 mm) was recorded in Makueni. Flowering activity indices in (June-August, 2018) were 64.97% and 69.6% in Tharaka and Makueni, respectively . Available P, average monthly rainfall and temperature had significant association with flowering and fruiting activity indices (p<0.05). The mean number of flowers and fruits per stem were significantly associated with soil available P, exchangeable Na and OC content (p<0.05). Though edaphic factors were not significantly associated with phenophase intensities of C. procera, average monthly rainfall and temperature were positively and negatively associated with phenophase intensities, respectively . Conclusion: Available P, exchangeable Na, available K and OC content noticeably affect phenological traits of naturally growing C. procera. Rains and temperatures are critical climatic factors affecting phenological traits of C. procera.

Keywords: phenophase intensity, activity index, phenology, Calotropis procera and edaphic

INTRODUCTION

Climate change is leading to acute food shortage, inadequate livestock forage and decreased income in arid and semi-arid lands (ASALs)¹. Therefore, there is need to develop environment friendly and conservation conscious techniques to increase communities’ resilience to climate change. Domesticating multipurpose trees and shrubs can significantly bring new opportunities for livelihood improvements². This is because domestication enhances provision of products and services from trees to increase productivity, combat malnutrition and adapt to anthropogenic climate change³.

Calotropis procera is among shrub species that can be domesticated in semi-arid regions. The shrub is ever green with deep and solid tap root, it is drought and salt tolerant, can grow in ecosystems with less than 1000 mm annual precipitation and temperature range of 20-30°C^4,5. The species can be used for medicinal and fodder purposes, while its genes can be used in genetic modification to enhance cotton fiber strengths⁶. However, the shrub has been reported to be having undesirable characteristics such as invasiveness in some parts of the world like Australia^7,8.

Different kind of research has established that its seeds and fruits can produce quality calotrope fiber that can be used in the textile industry. Compared to silk and cotton fiber, calotrope fiber has good stable lengths, fiber strengths, fiber uniformity ratio, fiber fairness and moisture absorption characteristics^9-11. Therefore, under proper management, C. procera can be ecologically, economically, culturally and socially important to ASAL communities. However, the phenological behavior of C. procera under different climatic and edaphic conditions has received limited research attention with most studies having been conducted in greenhouses^12,13. Lack of adequate information regarding this species makes it difficult to conclusively predict how climate change and changes in soil condition as a result of erosion and salination will influence the phenology of the species when domesticated¹⁴. Therefore, this study determined edaphic and climatic factors affecting the phenological traits of naturally growing C. procera in the semi-arid regions of Kenya.

MATERIALS AND METHODS

Research site: The study was carried out in the semi-arid regions of Tharaka and Makueni in the Eastern part of Kenya from June, 2018 to April, 2020. Tharaka lies between latitudes 00° 07' and 00° 26' S and longitudes 37° 19' and 37° 46' E, while Makueni lies between latitude 1° 35' and 3° 00' S and Longitudes 37° 10' and 38° 30' E. The two regions lie in the agro-climatic and eco-climatic zone V, which is characterized by low and unreliable rainfall, dispersed population, marginal agricultural lands and infertile soils¹⁵. The study was specifically conducted in lowland areas (altitude less than 600 m above sea level) that receive unreliable and poorly distributed rains of less than 500 mm per year and higher temperatures of up-to 40°C at certain periods¹⁶.

Research design: The study used a repeated measure research design by taking multiple measurements of the dependent variable on the same object over a period of time¹⁷. This was appropriate because the purpose of the study was to evaluate phenological plasticity of C. procera over a period of time under different climatic seasons and edaphic conditions.

Sampling procedures and sample sizes: Purposeful sampling technique was used in selecting research blocks (farms) with naturally growing C. procera and whose owners voluntarily allowed research to be conducted. In Makueni two blocks (Kyumani and Kyanguli) were selected while in Tharaka three blocks (Kathwana, Kilimangare and Kajiampau) were chosen. In each block, permanent main plots measuring (20×20 m) were marked using blue painted pipes. Each main plot was sub-divided into 15 permanent sub-plots measuring (5×5 m) that were demarcated using red painted pipes. Systematic random sampling technique was used in selecting sub-plots to be included in the study, where every third sub-plot was selected. The total number of sub-plots selected per plot was determined as explained by Dell et al.¹⁸, Eq. 1:

Where:

Since it was not known, it was estimated at 50% (0.5) as recommended by Dell et al.¹⁸:

Therefore, number of sub-plots per plot was:

All C. procera stems in sub-plots were numbered and included in the sample.

In each sub-plot, one pit was randomly dug to collect subsoil at 0-20 cm and deep soil at 20-40 cm. Soil samples at 0-20 cm and 20-40 cm from all sub-plots in a plot were mixed to form subsoil and deep soil composites, respectively . From each composite in the respective depth, one sample was picked and put into 2000 g well labeled bags for laboratory analysis.

Data collection
Phenology of naturally growing C. procera: Naturally growing C. procera stems with and without flowers and fruits were identified and counted. Activity index was calculated by dividing the number of stems with flowers or fruits by the total number of stems in a sub-plot. The number of flowers and fruits (green or ripe) per stem were counted and recorded. On every stem the total number of branches, number of branches with flowers and fruits were counted in the selected sub-plots. This was used to calculate the Phenophase Intensity (Pi) levels as indicated in Eq. 2 and 3¹³:

Where:

Soil properties: Soil samples were taken to Kenya Forest Research Institute (KEFRI) laboratory for analysis. Sample preparation and analysis of soil pH using pH meter, Electric Conductivity (EC) using conductivity meter, Organic Carbon (OC) using Walkley Black method, Phosphorus (P) using UV-Spectrophotometer and Magnesium (Mg), Nitrogen (N), Sodium (Na), Calcium (Ca) and Potassium (K) based on Atomic Absorption Spectrophotometer (AAS) were conducted according to Udelhoven et al.¹⁹.

Climatic factors: The geographical coordinates of the study area were used in obtaining rainfall and temperature data from National Aeronautics and Space Administration website.

Data analysis: Mixed ANOVA was used to determine statistically significant differences in the mean flowering and fruiting activity indices and phenophase intensities within research time points. Relationships between phenological traits with edaphic and climatic factors were established using linear and Poisson regression based on GEE. Analysis was conducted up-to a level that all remaining variables were significantly associated with phenological traits. Therefore, variables indicating insignificant association were removed from the model list-wise for the next analysis level.

RESULTS

Edaphic and climatic factors in Tharaka and Makueni semi-arid regions
Edaphic factors in Tharaka and Makueni semi-arid regions: Soil OC content and exchangeable Na at 0-20 cm soil horizon were 3.0% and 112.5 ppm in Tharaka and 3.08% and 75 ppm in Makueni, respectively , compared to 2.92% and 85 ppm in Tharaka and 2.63% and 74 ppm in Makueni, respectively at (20-40) cm soil depth (Table 1).

Climatic factors in Tharaka and Makueni semi-arid regions: The mean monthly rainfall of 143.83 mm and 160.37 mm were experienced in the period of (October, 2019 to February, 2020) in the semi-arid regions of Tharaka and Makueni, respectively (Fig. 1). Monthly average relative humidity of 60.42% and 61.52% and wind speed of 3.6 m/s and 3.07 m/s were experienced in (April-September, 2019) in Tharaka and Makueni, respectively (Fig. 2).

Factors Affecting Flowering and Fruiting Activity Indices of C. procera
Flowering and fruiting activity indices of C. Procera : Flowering activity indices of naturally growing C. procera decreased from 75.87% in Tharaka and 64.97% in Makueni in (June-August, 2018) to 48.05% in Tharaka and 50.48% in Makueni in (September-November, 2019) (Fig. 3). Similarly, fruiting activity indices decreased from 83.06% in Tharaka and 69.6% in Makueni to 42.71% and 43.64% over the same research time point, respectively (Fig. 3). Mixed ANOVA showed that mean flowering and fruiting activity indices varied significantly within research time points with (F_(3,267) = 27.211, p< 0.001, ηp2 = 0.234) and (F_(3,267) = 15.692, p< 0.001, ηp2 = 0.150), respectively .

Edaphic and climatic factors affecting flowering and fruiting activity indices: An increase in soil OC content, exchangeable Ca, exchangeable Na, soil EC, total N, exchangeable K and exchangeable Mg at 0-20 sm and 20-40 cm depth were neither increasing nor decreasing flowering and fruiting activity indices of C. procera significantly (p>0.05).

However, a unit increase in soil available P increased C. procera’s flowering activity index by 1.006 times in Tharaka and Makueni (Table 2). On the other hand, a unit increase in average monthly rainfall and temperature increased C. procera’s flowering and fruiting activity indices by 1.143 and 1.144 times, respectively (Table 3).

Factors affecting number of flowers and fruits
Number of flowers and fruits: The average number of flowers per flowering C. procera stem in Tharaka and Makueni decreased from 150 and 166 in (June-August, 2018) to 71and 80 in (September-November, 2019), respectively (Fig. 4). The highest number of fruits, 10 in Tharaka and 12 in Makueni were recorded in (June-August, 2018) (Fig. 4). Adjusted Greenhouse-Geisser, showed statistically significant variations in mean number of flowers (F_{(2.348, 744.261)} = 185.420, p<0.001, ηp2 = 0.369) and fruits (F_{(2.586,778.237)} = 269.464, p<0.001, ηp2 = 0.472) per flowering and fruiting C. procera stem within research time points.

Edaphic and climatic factors affecting number of flowers and fruits: It was established that a unit increase in exchangeable Na at 0-20 cm, OC content, available P, exchangeable Ca and exchangeable Na at 20-40 cm significantly increased the number of flowers by 1.002, 1.015, 1.048, 1.002 and 1.005 times, respectively (Table 4).

On the other hand, a unit increase in exchangeable Mg significantly reduced the number of flowers by 0.984 times (Table 4). On fruits, a unit increase in soil exchangeable Na at 0-20 cm, OC content, available P, exchangeable K, exchangeable Mg and exchangeable Na at 20-40 cm significantly increased the number of fruits by 1.005, 1.027, 1.049, 1.044, 1.044 and 1.009 times, respectively (Table 4).

On climatic conditions, a unit increase in monthly average rainfall and relative humidity significantly increased the number of flowers by 1.009 and 1.084 times, respectively, while a unit increase in monthly average temperature and wind speed reduced the number of flowers by 0.792 and 0.844 times, respectively (Table 5). On fruits, a unit increase in mean monthly rainfall, temperature and wind speed significantly increased the number of fruits by 1.056, 1.338 and 1.207 times, respectively (Table 5). Contrary, a unit increase in relative humidity significantly reduced the number of fruits by 0.794 times (Table 5).

Factors affecting phenophase intensity of C. procera in Tharaka and Makueni
Phenophase intensity of C. procera in Tharaka and Makueni: In (June-August, 2018), naturally growing C. procera in the semi-arid regions of Tharaka and Makueni recorded the highest flowering (77.57%) and (79.09%) phenophase intensities, respectively (Fig. 5). Mixed ANOVA showed statistically significant variations in mean flowering (F_(3,936) = 67.859, p<0.001, ηp² = 0.179) and mean fruiting (F_(3,93) p<0.001, ηp² = 0.043) phenophase intensities within research time points.

Edaphic and climatic factors affecting flowering and fruiting phenophase intensities: Parameter estimate (Table 6) indicates that a decrease of 0.999, 0.993, 0.994, 0.992, 0.997 and 0.956 times in C. procera’s flowering phenophase intensity as a result of a unit increase in soil pH, EC, OC, K, Ca and Na, respectively at 0-20 cm soil depth was not statistically significant (p>0.05). In addition, an increase of 1.002, 1.000 and 1001 times in C. procera’s flowering phenophase intensity as a result of a unit increase in soil N, P and Mg at 0-20 cm soil depth was not statistically significant (p>0.05) (Table 6). At 20-40 cm soil depth, a unit increase in soil pH, EC, N, P and Mg caused no statistically significant increase in C. procera’s flowering phenophase intensity of 1.004, 1.003, 1.005, 1.000 and 1.009 times, respectively (Table 6).

However, a unit increase in soil OC, K, Ca and Na at (20-40) cm soil depth led to a statistically no significant decrease of 0.996, 0.999, 0.998 and 0.999 times in C. procera’s flowering phenophase intensity (Table 6).

On fruiting, an increase of 1.007, 1.005, 1.009, 1.003, 1.001, 1.007and 1.006 times in C. procera’s fruiting phenophase intensities as a result of a unit increase in soil pH, EC, N, P, K, Mg and Na, respectively at 0-20 cm soil depth was not statistically significant (p>0.05) (Table 7). A unit increase in soil OC and Ca at 0-20 cm soil depth led to a statistically no significant decrease (p>0.05) in C. procera’s fruiting phenophase intensity by 0.994 and 0.998 times, respectively.

At 20-40 cm soil depth, soil pH, EC, N, OC, P, K and Ca led to a statistically no significant increase (p>0.05) in C. procera’s fruiting phenophase intensity by 1.018, 1.000, 1.001, 1.004, 1.004, 1.009 and 1.007 times, respectively (Table 7). On the other hand, a statistically no significant decrease (p>0.05) of 0.994 and 0.999 times in C. procera’s fruiting phenophase intensity as a result of a unit increase in soil Mg and Na, respectively at 20-40 cm soil depth (Table 7).

On the other hand, a unit increase in monthly average rainfall increased flowering and fruiting phenophase intensities by 1.014 and 1.012 times, respectively (Table 8). Contrary, a unit increase in monthly average temperature significantly reduced flowering and fruiting phenophase intensities by 0.981 and 0.965 times, respectively (Table 7). A unit increase in monthly average wind speed decreased fruiting phenophase intensity by 0.987 (Table 8).

DISCUSSION

Soils from Tharaka and Makueni were deficient in available P. This concur with Koala²⁰ that over 65.1% of soil samples from semi-arid regions are acutely deficient in total phosphorus. This deficiency in total phosphorus is as a result of imbalance in a number of biological and biochemical processes that are significantly influenced by soil organic matter, soil texture, biotic factors and abiotic characteristics of the region^21,22.

The study showed that the highest and lowest average monthly rainfall recorded was 160.37 mm and 52.55 mm per month, respectively for Makueni and 143.83 mm and 45.27 mm per month for Tharaka. These concur with Camberlin et al.¹⁶ that semi-arid regions of Kenya receive low, varied and unreliable rainfall. This is not different from other semi-arid regions which experience greater inter-and intra-annual rainfall variation²³. Average monthly temperature ranged from 25.78-28.15°C in Tharaka and 24.92-28.74°C in Makueni. These high temperatures may be attributed to high solar radiations, low cloud cover and their proximity to the equator²⁴. Wind speed variations were as a result of variations in temperature, cloud cover and earth’s revolution. According to Wooten²⁵ cloud cover affects temperature which creates pressure difference between places that eventually affects wind speed. There was noticeable relationship between edaphic factors with activity indices, number of flowers and fruits and phenophase intensities. This noticeable relationship between edaphic factors with phenological traits indicates that though C. procera can tolerate soils with low nutrient content due to its intensive root system that ensure reaching nutrients and moisture beyond 40 cm depth²⁶, soil conditions have slight impacts on the shrub’s phenology. Adequate availability of soil N content, available P, exchangeable Ca and OC content enhances the development of plant leaves and increases plant’s tolerance to other environmental stresses^27,28. Exchangeable Ca plays an important role in reducing the adverse effects of drought stress in plant crops²⁹. A healthy plant with improved photosynthesis ensures availability of carbohydrates for plants’ flowering and fruiting³⁰.

Deficiency of soil nutrients like available P, exchangeable Ca, available K and exchangeable Mg leads to stunted growth as a result of reduced photosynthesis and lower resistance to diseases^31,32. This condition leads to aborted flowers and fruits in plants by impairing female reproductive organs and reduces pollen grain formation and viability especially under high saline and drought conditions^33-35. Exchangeable Mg is essential for chlorophyll a and b in light energy and synthesis of both in plants³⁶.

Soil pH and EC were not associated with phenological traits of C. procera. This was because the shrub has adaptive avoidance mechanism to salinity and pH stresses^37,38. According to Fekry et al.³⁹ high salinity inhibit growth of plants like date palm hence the need to alleviate its effects on growth and fruiting. Gulzar et al.⁴⁰ recommends that a combination of nitrogen and phosphorus fertilizers can improve growth and productivity of plants that are salt stressed.

The significant association between phenological traits with average monthly rainfall and temperature concur with studies like Moore and Lauenroth¹⁴ that temperature and rainfall influences phenological events especially in ASALs. This is because phenology development requires optimal temperature and adequate moisture that is influenced by rainfall⁴¹. Temperature and precipitation influences pollen and ovule viability and affects visitation by pollinators^42-44. Extreme temperature and precipitation reduces photosynthetic activity of C. procera as they affect opening and closing of plant’s stomata, hence reducing availability of flowering and fruiting energy in plants. In addition, extreme environmental stresses including high temperature and low rainfall makes plants susceptible to pathogens and diseases⁴⁵.

However, the association was weak with low odd ratios because other factors like plant size especially in terms of crown diameter and genetic composition influences phenological traits like number of flowers and fruits⁴⁶. Large crowns provide more space for flowers and fruits. In terms of genetics, though C. procera can withstand harsh climatic conditions like high temperatures and low rainfall⁵, the shrub experiences low fertility rates, high drop of floral buds and flower abortion after anthesis regardless of prevailing conditions⁴⁷.

Wind speed was slightly associated with phenology of C. procera negatively. High wind speeds causes traumatic flower and fruit fall before maturity. It also discourages flower visitation by pollinators by desiccating flower parts, making them unattractive, hence lowering fertilization rates⁴⁸. However, high wind speed increases the chances of self-pollination assisted by wind⁴⁸.

Relative humidity affects phenology of plants indirectly by affecting pollination, photosynthesis and disease occurrence⁴⁹. Low relative humidity increases transpiration, leading to water deficit for photosynthesis⁵⁰. However, high relative humidity impedes dispersal of pollen grains from anthers and increase disease instances by favouring fungal growth⁴⁹.

Phenological traits of C. procera peaked in (June-August, 2018 and troughed in (September-November, 2019) in Tharaka and Makueni. This concur with Sobrinho¹³, Paradiso and Pascale⁵¹ and Moustafa and Sarah²⁶ that C. procera show peak and low phenology traits at different times of the year depending on prevailing environmental conditions like precipitation and temperature.

CONCLUSION

Semi-arid regions of Tharaka and Makueni in Kenya experience low monthly rainfalls, medium temperatures and wind speed that vary from time to time. Soils in Tharaka were deficient in available P and exchangeable K while those of Makueni were deficient in available P. Flowering activity index of C. procera requires adequate supply of soil available P while an increasing number of flowers per stem requires optimal supply of soil exchangeable Na, OC content, available P and exchangeable Mg. Optimal fruit production of C. procera fruits requires adequate supply of soil exchangeable Na, OC content, available P, available K, exchangeable Mg and exchangeable Ca. Enhancing phenological traits of C. procera requires optimal rains, temperatures and wind speed.

SIGNIFICANCE STATEMENT

This study discovered that phenological traits of C. procera are influenced by both edaphic and climatic factors. Soil properties such as soil exchangeable Na, OC content, available P and exchangeable Mg increased the production of flowers and fruits. Similarly, average monthly rainfall and temperature are critical factors influencing phenological traits. This information is important when introducing the plant from the wild to on farm cultivation. This study will help the researchers to uncover the critical areas in determining reproductive successes of the plant in its environment for the purpose of domestication. Thus a new theory on the success of C. procera domestication may be arrived at for a sustainable supply of fiber for the growing textile industry in Kenya.

ACKNOWLEDGMENT

We thank all farm owners that voluntarily allowed access to their farms for research. This research was financially supported by German Academic Exchange Service (DAAD) under ICRAF- DAAD collaboration (DAAD-1157).

REFERENCES